Strength members, reinforcing members, and structural members made of metal and having a bent shape which are used in automobiles, various types of machines, and the like need to have a high strength, a low weight, and a small size. Up to now, these bent members have been manufactured by methods such as welding of stamped parts, punching of thick plates, and forging. It is difficult to further reduce the weight and size of bent members which are manufactured by these methods.
Non-Patent Document 1 discloses the manufacture of this type of bent member by the so-called tube hydroforming technique. Page 28 of Non-Patent Document 1 discloses that tube hydroforming has the problems of developing materials for blanks to be worked and increasing the degree of freedom of shapes which can be formed. Further development in the future is necessary in order to manufacture this type of bent member by tube hydroforming.
The present applicant disclosed a bending apparatus in Patent Document 1. FIG. 7 is an explanatory view schematically showing such bending apparatus 1.
As shown in FIG. 7, the bending apparatus 1 manufactures a bent member 7 made of steel by carrying out bending of a steel tube 2, which is supported by a pair of support means 3,3 so as to movable in its axial direction, in the manner described below on the downstream side of the support means 3, 3 while feeding the steel tube 2 in the direction of the arrow by an unillustrated feed device from the upstream side to the downstream side.
A high frequency heating coil 4 disposed on the downstream side of the support means 3, 3 rapidly heats the steel tube 2 to a temperature range in which local quench hardening is possible. A water cooling device 5 disposed on the downstream side of the high frequency heating coil 4 rapidly cools the steel tube 2. A movable roller die 6 having at least one set of roll pairs 6a, 6a which can support the steel tube 2 while feeding it varies its position three-dimensionally (or in some cases two-dimensionally) to apply a bending moment to the high temperature region 2a of the steel tube 2. Therefore, the bending apparatus 1 can manufacture a bent member 7 with high operating efficiency.
Non-Patent Document 2 discloses a technique for dieless bending of a metal tube by which a metal member having an elliptical cross section is continuously fed, and while supporting the metal member being fed by a support device so that the member can move, rapid heating of the metal member with a high frequency heating device disposed downstream of the support device and rapid cooling of the metal member with a water cooling device disposed immediately downstream of the high frequency heating device are performed, and while gripping the metal member with an arm which can rotate two-dimensionally and which is disposed downstream of the rapid cooling device, the arm is rotated to apply a bending moment to the high temperature region of the metal member. The technique disclosed by Non-Patent Document 2 does not form a complicated shape which is bent two-dimensionally or three-dimensionally like the invention disclosed in Patent Document 1, and it is also not intended to produce an increase in strength by quench hardening.
Non-Patent Document 2 discloses the dimensions of each portion of a bent member in which an elongated flat metal blank having a hollow closed cross-sectional shape and an integral structure in the lengthwise direction is bent to a two-dimensional shape having a constant bend radius by carrying out local rapid heating and cooling of the blank while feeding it in its lengthwise direction.
According to the investigations of the present inventors, the bending apparatus disclosed in Patent Document 1 and the technique disclosed in Non-Patent Document 2 can manufacture a bent member having a bent portion with a bend radius which is 1 to 2 times the diameter of a metal tube (in the case of a rectangular cross section, the length of a side in the bending direction of a metal tube). However, with such technique, it is difficult to perform mass production at a low cost of bent members having a bent portion with an extremely small bend radius (such as at most 1-5 times the wall thickness) which are commonly used in automotive parts such as components of automotive bodies or suspensions or chassis.
FIG. 8 is an explanatory view showing the change in the dimensions of the cross section of a hollow member before and after the bending disclosed in Patent Document 1 and Non-Patent Document 2. FIG. 8 shows the case in which bending is first carried out with a bend radius R1 and then is carried out in the opposite direction with a bend radius R2.
As shown in FIG. 8, when bending is carried out by applying a bending moment to a profiled tube 8a having a rectangular cross section with a width W0 and a height H0, the width of the resulting bent member 8b, which is a product after the completion of bending, is W1, which is smaller than the width W0 before bending by a small amount ΔW (W1=W0−ΔW). In addition, the height HB of the product on side B, which is the inner peripheral side of bending, is increased by a minute amount ΔH with respect to the height H0 before bending (HB=H0+ΔH), while the height HA on side A, which is the outer peripheral side of bending, decreases by a minute amount ΔH′ compared to the height H0 before bending (HA=H0−ΔH′).
As disclosed in Non-Patent Document 2, when a bent member 8b is manufactured by bending of an elongated flat metal blank 8a having a hollow closed cross-sectional shape and an integral structure in the lengthwise direction, in a portion where bending is performed, the width and height of a cross section vary from the dimensions before bending. Wrinkles develop on the inner peripheral side of bending and the cross-sectional shape deforms, and as a result, the dimensional accuracy of the bent member 8b decreases. The extent of the decrease in dimensional accuracy depends on the dimensions of the blank 8a before bending (the width W0, the height H0, the wall thickness t, etc.) as well as the bending conditions (the bend radius R, the heated width b, etc.).
If it is attempted to manufacture a bent member having a bent portion with an extremely small bend radius R (such as at most 1-5 times the wall thickness of a hollow member) by the bending apparatus disclosed in Patent Document 1 or the technique disclosed in Non-Patent Document 2, the dimensional accuracy of the resulting bent member greatly decreases, and it is not possible to manufacture a bent member having excellent dimensional accuracy.
Non-Patent Document 3 discloses a technique for manufacturing a stainless steel tube which is formed into the shape of a crank having a bent portion with an extremely small radius by cold working. According to this technique, a SUS 304 stainless steel tube is disposed inside a sectional die which is split into an upper section and a lower section and which constrains expansion of the steel tube, shear deformation in a plane at 45 degrees to the axis of the steel tube is developed by moving the outer die in a direction perpendicular to the steel tube while applying an internal pressure p to the steel tube and pressing the end surface of the tube in the axial direction with a load F, and the location where deformation takes place is continuously moved as the outer die moves.